The present invention, in some embodiments thereof, relates to energy conversion and, more particularly, but not exclusively, to a direct liquid fuel cell system, which utilizes ammonia borane, hydrazine or derivatives thereof as fuel, and to applications employing such a fuel cell system.
Fuel cells (FC) are well-known and widely studied electrochemical devices that enable the conversion of the chemical energy of fuels directly into electrical energy, thereby avoiding the Carnot cycle limitations and loss of efficiency associated with combustion-related engines. Contemporary hydrogen-oxygen fuel cells can be 50%-65% efficient in practice. Such efficiency values are far higher than typical values for internal combustion engines. Fuel cells are generally characterized by high theoretical energy density—4 to 8 KWh/kg—much higher than conventional batteries, which are limited to specific energies of 150-200 Wh/kg. Since the power source is the weakest link in many expanding industries, fuel cells is one of the main thrusts in the field of power engineering.
Over the past four decades, several different types of fuel cells (typically categorized by the electrolyte they use), most being hydrogen-based fuel cells, have been developed for a wide range of applications. Although present-day fuel cells have proven generally useful for their intended purposes, they all suffer from inherent deficiencies which detract from their utility and desirability. Some of the most handicapping limitations of current fuel cell technologies are related to the use of expensive noble metal catalysts, limited catalyst structural and functional stabilities, and noble metal catalyst low tolerance for even very low concentrations of carbon dioxide (few ppm at 80° C.) which inherently limits fuel cell operation.
The present inventors have previously uncovered that fuel cell systems which are operable by ammonia borane, hydrazine, or derivatives thereof, act efficiently also in the presence of non-noble metals and/or non-metallic substances. The present inventors have uncovered that a copper-based catalyst can be efficiently used for the anode. See, WO 2010/055511 and WO 2010/055512, which are incorporated by reference as if fully set forth herein.
Ammonia borane (or borazane) is characterized by an electrical capacity of 5200 Ah/kg, energy density of 8400 Wh/kg (as NaBH4) and hydrogen content of 19% (w/w). AB is stable in aqueous solutions at pH≧6.5, in contrast to BH4−. The standard potential of reduction (E0) of Ammonia-borane is −1.216 V (see, equation 2 below).NH3BH3+6OH−→BO2−+NH4++4H2O+6e− E0=−1.216 V  (1)
Yao et al. [Journal of Power Sources 2007, 165, 125; referred to herein throughout as Zhung] described a fuel cell consisting of 0.5 M AB (2 M NaOH)—Ag catalyst/air/MnO2 catalyst. The cell produces an open circuit potential (EOCP) of −1.15 V, a current of 1 mA/cm2 for EW=0.9 V, a current of 2 mA/cm2 for EW=0.8 V and a current of 10 mA/cm2 for EW=0.4 V.
Zhang et al. [J. Pow. Sour. 2007, 168, 167; referred to herein throughout as Xu-1] describe a fuel cell consisting of AB (2 M NaOH)-air fuel cell using Pt catalyst (0.15 mg/cm2 for anode and cathode). In this fuel cell, thiourea (1 mM) was added to the background electrolyte in order to prevent fuel spontaneous hydrolysis (decomposition). The cell produces a current of 24 mA/cm2 (EW=0.8 V) at room temperature. Zhang et al. [J. Pow. Sour. 2008, 182, 515; referred to hereinthroughout as Xu-2] further described fuel cell that consists of anode—0.5 M AB (2M NaOH)/Pt-0.9 mg/cm2/cathode Pt-1.3 mg/cm2, humidified O2. Pump was used for fuel supply and fan was used for air (O2) supply. The cell produces a current of 50 mA/cm2 at EW=0.75 V (EOCP=−1.08 V).
Further background art includes a review by Demirchi and Miele [Energy & Environmental Sci, 2009, DOI 10. 1039/b900595a)], in which sodium borohydride-based fuel cells vs. ammonia borane-based fuel cells are discussed.
Hydrazine is considered a hazardous compound, both in its pure form (as N2H4) and as a monohydrate (N2H4.H2O). Yet, hydrazine is non-explosive and non-toxic in diluted aqueous solutions. Moreover, several hydrazine salts, such as, for example, N2H4.H2SO4 are reported as prospective anticancer drugs [see, for example, Upton et al. Tren. Pharm. Sci. 2001, 22, 140-146].
The basic sources of hydrazine in nature are unlimited (N2 and H2) and the recycling of hydrazine from its basic elements (N2 and H2) is relatively simple. In addition, hydrazine decomposition results in byproducts, nitrogen (N2) and water (H2O), which are ecologically friendly.
The electrochemical oxidation of hydrazine in a basic solution produces four electrons, nitrogen gas (N2) and water, as presented in Equation 1 hereinbelow:NH2—NH2+4(OH)−→N2+4H2O+4e−  (2)
The standard potential of hydrazine oxidation)(EO) corresponds to −1.21 V, its theoretical specific electrical capacity corresponds to 3.35 KAh/kg and its specific theoretical power (W) corresponds to 4.05 KWh/kg (3,350·1.21).
The electrochemical properties of hydrazine in alkaline solutions were investigated in the last three decades using different metal catalysts such as platinum (Pt), palladium (Pd), Nickel (Ni), cobalt (Co), gold (Au), silver (Ag) and mercury (Hg).
Amongst the tested metals, Co, Ni and especially Pt-group metals (PGM) were found to perform as the best catalysts for the electro-oxidation of hydrazine.
U.S. Patent Application No. 2008/0145733, by Asazawa et al., discloses fuel cells operated using hydrazine and other amine and hydrogen containing compounds as fuel and a cobalt-containing catalyst layer.
Ghasem Karim-Nezhad et al. [Electrochimica Acta 54 (2009) 5721-5726] disclose copper (hydr)oxide modified copper electrode for electrocatalytic oxidation of hydrazine in alkaline media. The modified electrode showed improved stability to corrosion and an improved electrochemical performance (a negative shift of about 120 mV as compared to a bare copper electrode). The disclosed Cu modified electrode, however, operates at a working potential of +0.2 V, which is not suitable for fuel cell applications (for fuel cell application an anode potential of at least −0.5 V is needed).
Some background art concerning interactions between hydrazine and copper(II) ions includes Zhiliang Jiang et al. [Anal. Chem. 2008, 80, 8681-8687], which report that Cu(II) ions serve as catalysts for homogenous Hz decomposition.
Fuel cell systems operating with hydrazine as a fuel and various oxidants have been taught. Commonly used oxidants include, for example, air (for oxygen supply), nitrous acid, and hydrogen peroxide.
U.S. Pat. No. 3,811,949 discloses a hydrazine-based fuel cell system comprising metal alloys (e.g., amalgams) as catalysts and oxygen as the oxidant. The main disadvantage in this fuel cell is the use of dangerous mercury-containing electrode.
Electrochemical hydrazine sensors were also developed in the last decades [see, for example, Abbaspour and Kamyabi; J. Electroanal. Chem. 2005, 576, 73-83; Ozoemena and Nyokong; Talanta, 2005, 162-168; Karim-Nezhad and Jafarloo, Electrochimica Acta; 2009, 54, 5721-5726]. These electrochemical sensors utilize as catalysts noble metals, transition metals, organic and inorganic complexes, oxides, metal phthalocyanides, metal porphyrines, and more.
Several publications have reported that CuSO4/Cu(II) is an effective promoter for homogenous oxidation of hydrazine [see, for example, J. Ward, J. Am. Chem. Soc. 1976, 98, 7; J. Corey. J. Am.; Chem. Soc. 1961, 83, 2957; J. Rempel, Appl. Catalysis A: General, 2004, 263, 27; Z. Jiang. Anal. Chem. 2008, 80, 8681].
WO 2010/055512 describes fuel cell systems comprising an anode and/or cathode which comprise a non-noble metal (e.g., copper) or a non-metallic substance (e.g., an iron electron-transfer mediating complex) as a catalyst. The disclosed fuel cell systems are operable by ammonia borane or derivatives thereof as fuel and utilize a peroxide as an optional oxidant. Exemplary systems include an anode comprising a copper catalyst layer. Optionally, the cathode comprises a metallic complex as a catalyst layer.
WO 2010/055511 also describes fuel cell systems comprising an anode and/or cathode which comprises a non-noble metal (e.g., copper) or a non-metallic substance (e.g., an iron electron-transfer mediating complex) as a catalyst, which are operable by hydrazine or derivatives thereof as fuel. Exemplary systems include an anode comprising copper nanoparticles as a catalyst layer.
Additional background art includes U.S. Pat. Nos. 6,562,497; 6,554,877; 6,758,871, US 2003/0207157, and Yamada et al., Electrochemistry Communications, 2003, 5, 892-896.